1. FIELD OF THE INVENTION
The present invention relates to a fuel control system for an internal combustion engine, which controls fuel supply rate on the basis of intake air flow detected by using a Karman vortex street.
2. DISCUSSION OF BACKGROUND
The conventional fuel control system for an internal combustion engine measures an intake air flow in a suction stroke with a vortex flowmeter which generates a signal of a frequency proportional to the intake air flow, obtains a fuel injector driving period by multiplying the measured intake air flow by a correction coefficient and adding a dead time calculated on the basis of the voltage of the battery, i.e., a power supply, to the product, and drives the fuel injectors for the injector driving period to control the fuel supply rate.
The prior art will be described with reference to FIG. 5 showing a vortex flowmeter applied to a fuel control system in accordance with the present invention.
An ultrasonic transmitter 4 and an ultrasonic receiver 5 are disposed opposite to each other with a flowmeter 1 having a vortex generator 2 therebetween. An ultrasonic oscillator 6 drives the ultrasonic transmitter 4 so that ultrasonic waves are propagated across a Karman vortex street 3 generated after the vortex generator 2.
The ultrasonic waves propagated across the Karman vortex street 3 undergo phase modulation and are received by the ultrasonic receiver 5. A waveform shaping circuit 8 shapes the waveform of the received signal and gives an output signal to a phase comparator 9.
On the other hand, the output of the ultrasonic oscillator 6 driving the ultrasonic transmitter 4 is applied to a voltage-controlled phase deviating circuit 7.
The voltage controlled phase shift circuit 7 maintains the high frequency stability of an ultrasonic oscillation frequency signal and controls only the phase shift. The voltage controlled phase shift circuit 7 applies the output of the ultrasonic oscillator 4 to the phase comparator 9 after shifting the phase of the same.
The phase comparator 9, the ultrasonic oscillator 6, the voltage-controlled phase shift circuit 7 and a loop filter 10 constitute a phasing loop. Indicated at 11 is a low pass filter.
The phase comparator 9 compares the phase of the output of the waveform shaping circuit 8 and the phase of the output of the voltage-controlled phase shift circuit 7, and then applies signal presenting the result of comparison to the loop filter 10. The loop filter 10 removes the unwanted frequency component of the result of comparison.
The voltage controlled phase shift circuit 7 controls the phase shift of the output signal of the ultrasonic oscillator 6 according to the output voltage of the loop filter 10, and then gives an output signal to the phase comparator 9.
The output of the phase comparator 9 is given directly to the loop filter 10, and through the low-pass filter 11 to a first variable-frequency filter 12.
The first variable-frequency filter 12 is high-pass filter which passes the high-frequency components of the output signal of the low-pass filter to a second variable-frequency filter 13.
The second variable-frequency filter 13 is a low-pass filter which passes the low-frequency components of the input signal to a waveform shaping circuit 14.
Referring to FIG. 6, the first variable-frequency filter 12, i.e., a high pass filter, removes noise components of frequencies below a lower limit frequency f.sub.L. The second variable-frequency filter 13, i.e., a low-pass filter, removes engine noise components of frequencies above an upper limit frequency f.sub.U. Consequently, a frequency band between the upper limit frequency f.sub.U and the lower limit frequency f.sub.L is a passband for both the first variable frequency filter 12 and the second variable-frequency filter 13.
The engine noise is a noise of a comparatively low frequency generated by the pulsation of air flow, a noise of a low output frequency resulting from a so-called swishy sound generated as air flows through an air valve, namely, a noise of a comparatively high frequency, or a noise of a high output frequency generated by the supercharger.
Since a noise generating region is variable and the air flow varies according to the momentarily variable operating mode of the engine, the frequencies of vortices are distributed in a comparatively wide band width. Accordingly, the first variable-frequency filter 12 and the second variable-frequency filter 13 are employed in combination.
A waveform shaping and amplifying circuit 14 shapes the waveform of a vortex frequency signal passed by the first variable-frequency filter 12 and the second variable-frequency filter 13 and amplifies the same to provide a vortex frequency signal.
At the same time, a frequency to-voltage converter (hereinafter, referred to as "f-V converter") 15 converts the vortex frequency signal into a voltage corresponding to the frequency. The respective passbands of the first variable-frequency filter 12 and the second variable-frequency filter 13 are controlled on the basis of the voltage provided by the f-V converter 15.
Thus, the respective passbands of the first variable-frequency filter 12 and the second variable-frequency filter 13 are varied, so that the width of the passband indicated by a hatched area in FIG. 6 is varied.
The conventional fuel control system for an engine thus constituted is unable to achieve accurate fuel control when applied to an engine equipped with a supercharger, because the waveform of a vortex signal is disturbed by ultrasonic noises generated by the supercharger in measuring intake air flow by the vortex flowmeter.
Particularly, when the throttle valve is shut suddenly, the intake air flow diminishes, whereas the rotating speed of the rotor of the supercharger does not drop rapidly due to the inertia of the rotor. Accordingly, the SN ratio of the waveform of a signal at the output terminal of the second variable frequency filter 13 is very low as against a signal at the input terminal of the first variable-frequency filter 12.
When a signal of such a waveform is applied to the second variable frequency filter 13, the variable-frequency filter 13 provides an output signal of a very high frequency. Consequently, during deceleration in which the throttle valve is shut, excessive fuel is supplied to the engine, and the overrich mixture deteriorates the operation of the engine during deceleration.